In recent years, plastics have become widely used in the field of containers for food products, beverages, cosmetics, and for numerous other applications. Polyester resins, such as polyethylene terephthalate ("PET"), have become very popular because they can be blow molded into thin-walled containers having excellent physical properties and characteristics.
Conventional blow molding of plastic containers typically involves two steps. In the first step or phase, an intermediate article, or preform, is formed. The second phase involves biaxially orienting the preform into the final article by a process commonly referred to as blow molding, or stretch blow molding. Many of the properties of the plastic material are best realized once the container has been biaxially oriented and blow molded. Quite commonly during such processing, the neck portion of the preform is used as a mounting portion for the blowing mold and the finish or threaded area of the preform is not specifically heated. Also, because that segment of the preform is not fully oriented, it will not exhibit the full benefit of properties resulting from the biaxial blow molding method. For instance, PET containers in which the neck region has not been subjected to, or more fully subjected to, an orientation often exhibit reduced thermal resistance to deformation.
While the previously mentioned two-step process is frequently used to produce large volumes of containers for a variety of applications, in a number of specialized applications, the product content must be filled at elevated temperatures to ensure proper sterilization. For instance, beverages which are pasteurized, such as some European drinks, are bottled in a range of 148.degree. to 170.degree. F. Drinks which include a portion of fruit juice are typically hot-filled in a range of 170.degree.-185.degree. F. Moreover, some fruit drinks and the like, require even higher hot-fill temperatures, i.e., 190.degree. to 200.degree. F. and above, to achieve an appropriate level of purification.
Elevated filling temperatures pose a challenge in constructing plastic containers because thermoplastic materials are known to increase in plasticity with temperature over time. Exposure of the container to the contents at higher fill temperatures can cause portions of the article to soften and deform, making it more difficult to maintain the container's structural integrity. This is especially true when longer periods of time are involved and/or when the temperature of the hot-fill product exceeds the glass transition temperature of the plastic, i.e., the temperature at which plastic changes from a solid to a soft, rubbery state. For reference, the glass transition ("T.sub.g ") or softening temperature of PET is approximately 170.degree. F.
When the filling temperature approaches or exceeds transition temperature of the polymer, container manufacturers often employ additional thermal conditioning techniques to help avoid the associated thermal shrinkage and resulting distortion. In a number of applications, it becomes necessary to stiffen specific portions of the container to prevent an unacceptable amount of deformation. This is especially true for the neck region when hot-filling product contents at about 185.degree. to 200.degree. F. or higher, or when causing a closure roll-on die or a lugged neck finish to apply a closure means to the final container.
In the case of PET containers, non-discriminate heat treating of the entire container would induce spherulitic crystal growth in non-molecularly oriented portions of the container. The resulting container would have opaque, brittle portions, and would be commercially undesirable. As such, controlling the crystallization process is a basic consideration in determining the physical properties of the container.
The prior art discloses the practice of incorporating a heat treatment process to stiffen the neck section of preforms and/or containers. Essentially, heat treatment is used to induce crystallization at the neck portion of the preform or container in an effort to increase thermal resistance to deformation. The primary drawback with such methods is that it generally takes a significant amount of time to sufficiently heat treat the thickness of the neck portion, it being one of the thickest portions of the preform. Because this process can be costly and time-consuming, there exists a need to develop techniques for improving the properties of the upper, or neck, portion of preforms and the resultant containers in a more commercially efficient manner.
In addition to heat treating processes, manufacturers of plastic containers often attempt to take advantage of the use of multiple layers of plastic materials. Such multi-layer containers, i.e., those having multiple layers throughout all or portions of the article, often prove to be more desirable than their mono-layer counterparts for a number of hot-fill applications. This is often because the individual layers of a multi-layer structure may provide independent benefits and the layers can be selected from materials to better optimize functional characteristics. Multi-layer containers have found an increasing role in the manufacture of plastic containers and are commonly known to those skilled in the art.
Therefore, by employing processes which take advantage of both the multi-layered structure along with limited and controlled crystallization, one skilled in the art can best adapt the physical structure of the container to meet the needs of a given application.